The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (very high confidence). There are, however, differences between simulated and observed trends over periods as short as 10 to 15 years (e.g., 1998 to 2012).

From 1998 through 2012, the Met Office estimated that global surface temperatures had warmed by about 0.06°C, whereas the average climate model projection put the value at closer to 0.3°C. This apparent discrepancy only represented a tiny fraction of overall global warming, and over a short enough period of time that the internal noise of the climate system could be having a significant influence, but it was nevertheless a challenge for climate scientists to explain the precise causes of the difference.

However, research published since the release of the IPCC report has gone a long way to explain the discrepancy, and it turns out the climate models are doing a pretty good job after all.

Measurement Biases Exaggerated the Discrepancy

One important paper published late last year by Kevin Cowtan and Robert Way found that the discrepancy wasn’t nearly as large as believed. Climate scientists have known that the surface temperature record is incomplete because of gaps between instrument stations, especially in the Arctic. The Arctic happens to be the fastest-warming region on Earth, and so if it’s excluded from the surface temperature record, the record will have a cool bias.

Cowtan & Way took two separate clever approaches to address this issue, using statistics and satellite measurements to fill in the gaps. Both methods yielded similar results, showing that the Met Office estimates were accurate up to 1998, but underestimated the warming since 1998 by more than half, due in large part to the particularly rapid warming in the Arctic during that time.

The Cowtan & Way estimates put the global surface warming from 1998 through 2012 at about 0.17°C, explaining nearly half of the apparent difference between measurements and the average climate model projections. But the other half still needed to be explained.

A recent study led by Matthew England showed that unprecedented trade wind strength has invigorated the circulation of the Pacific Ocean, causing more heat from the surface to be mixed down into deeper ocean layers, while bringing cooler waters to the surface. Papers by Kosaka & Xie and James Risbey and colleagues showed that when climate models accurately reflect ocean cycles, their global surface temperature simulations are also accurate.

At the same time, solar activity has declined and volcanic activity risen since 1998. Volcanoes pump aerosol particulates into the atmosphere that cause short-term cooling, and a decline in the amount of solar energy reaching the Earth’s surface also has a cooling effect. Another recent paper led by NASA climate modeler Gavin Schmidt showed that when incorporating the observed changes in these two factors and the Cowtan & Way corrections, climate models did a good job reproducing the observed global surface warming.

With these internal (ocean cycle) and external (solar and volcanic activity) explanations, we were still left with the question – how much was each responsible for the global surface warming slowdown?

Huber & Knutti Show it’s about Half and Half

A new paper published by Huber & Knutti (good explanation here) has found that from 1998 to 2012, ocean cycles caused about 0.06°C global surface cooling, the sun caused 0.04°C, and volcanoes caused 0.035°C cooling.

Putting it all together, we have 0.17°C observed surface warming according to Cowtan & Way, and 0.13°C cooling from natural influences. If those natural factors hadn’t caused cooling since 1998, we would have seen 0.3°C global surface warming, right in line with climate model projections.

The figure below from the Huber & Knutti paper illustrates the point nicely. The dotted orange and solid black lines show the unadjusted average model projection and measured global surface temperatures, respectively. The solid orange and dashed black lines show these estimates adjusted to reflect the changes in ocean cycles, solar output, volcanic activity, and surface temperature measurement biases.

Huber & Knutti show that when climate models account for these short-term natural changes, their temperature projections are right on the money.

The bad news is that we can’t yet predict changes in ocean cycles, solar output, or volcanic activity accurately, so it’s going to be hard to improve short-term climate model projections. The good news is that these factors make little difference in long-term climate changes or predictions. Solar and volcanic activity tend to be relatively stable, and will barely make a dent in human-caused global warming. Positive and negative phases of ocean cycles cancel each other out over the long-term.

What about the Future?

Long-term global warming is dictated by the Earth’s climate sensitivity. Huber & Knutti showed that the average climate model output (with an average climate sensitivity of about 3°C global surface warming at equilibrium from a doubling of atmospheric carbon dioxide) produces accurate global temperature predictions.

They also tweaked the sensitivity of a climate model of intermediate complexity, and again showed when the aforementioned internal and external factors are accounted for, models with an equilibrium climate sensitivity of 3°C do the best job reproducing global surface temperature changes. When tweaked to have equilibrium climate sensitivities of 2°C and 4°C, the model surface temperature simulations were significantly less accurate.

All of the available evidence indicates that the global surface warming slowdown is temporary, and will only last as long as solar activity keeps falling, volcanic emissions keep rising, and until ocean cycles switch phases.

It’s only a matter of time until global surface warming once again accelerates. And until we get our carbon pollution under control, we’ll remain on a path towards highly dangerous and costly climate changes.